Detection Methods And Distribution Characteristics Of Microplastics In Typical Organic Solid Wastes

Land application of organic solid waste recycling products is the fundamental way of resource utilization,and it is also an important source of microplastics（MPs）in the soil system.Researching on the distribution characteristics of MPs in organic solid waste is of great significance for clarifying the migration and transformation mechanism of MPs in different environment and solid waste management.Organic solid waste samples are rich in complex organic matter.The extraction of MPs from organic solid waste recycling products is extremely challenging.At present,accurate and reliable analysis methods are still lacking,which restricts the comparability of related research.In this study,by optimizing the organic matter digestion method and the MPs separation method,a comprehensive method for the analysis of MPs in organic solid waste and its resource products is constructed.The constructed method is used to extract,analyze and identify MPs in typical organic solid waste（perishable waste,livestock manure,sludge）and its recycling products.The shape,color,particle size distribution and polymer types of MPs and other distribution characteristics were studied,MPs pollution levels and ecological risks were evaluated in this research.This study provides a scientific basis for the ecological risk assessment in the process of organic solid waste management and resource utilization.The main conclusions of the study are as follows:（1）Different digestion pretreatment’s effects on organic matter removal efficiency and MPs particles were explored.Acid digestion and alkaline digestion influencesurface degradation of some MPs particles and even molecular structure changes,which will affect subsequent analysis and identification.Oxidative digestion and enzymatic digestion have little effect on the interference of MPs particles.Among all the digestion schemes,30%H₂O₂ is the digestion scheme with the least damage to MPs and the highest organic matter removal rate（75%-87%）.The temperature threshold of H₂O₂ digestion is 60℃,and the time threshold is 36 hours.PA is the most sensitive to the pretreatment process and can be used as an indicator polymer in response to the influence of the digester.（2）The effects of density separation,oil separation and magnetic separation on the recovery of MPs in different environmental samples were compared.The type of environmental sample has little effect on the recovery effect in the recycling experiment.The recovery rates of MPs are closely related to the type,size and shape of MPs.The recovery rates of MPs decrease as the size decreases.Among MPs of the same particle size,the recovery rates of fragments and granular MPs are easier to recover than fiber and film MPs.The total recoveries of MPs in samples riched in organic matter are as follows:two-stage density separation>magnetic extraction>oil extraction.A comprehensive analysis method of MPs pollution in organic solid waste was constructed with H₂O₂ pretreatment-density separation-stereoscopic/FTIR/SEM.（3）The distribution characteristics of MPs in organic solid waste and its recycling products were systematically investigated.The shapes of MPs in organic solid wastes and its recycling products are mainly fibre and film;the colors of MPs are mainly black,red,blue and green;the types of polymers of MPs are PE,PP and PET.The surface of MPs extracted from environmental samples generally has protrusions,depressions,cracks or holes.Microbes are observed to adhere to the surface of some MPs.（4）The abundance of MPs in the perishable domestic waste is about 5.78-6.33×10³N·kg^-1.The MPs abundance in the compost is in descending order of dynamic composting>semi-dynamic composting>static composting.Because of mechanical abrasion,chemical oxidation,and biodegradation during the composting process,large-particle size plastics may age and degradate,and may even be broken down into smaller pieces which increased the ecological risk.The abundance of MPs in food waste is11.78±0.88×10³ N·kg^-1,and the sample contains non-MPs impurities such as quartz,celery and stainless steel that may interfere with the study of MPs pollution characteristics.The abundance of MPs in pig manure is 2.22±0.16×10³ N·kg^-1,and the abundance of MPs in cow manure is 1.89±0.31×10³ N·kg^-1.The MPs abundance of the sludge in this study(13.83-23.83×10³ N·kg^-1)is equivalent to the national average level of pollution in the related research(22.7±12.1×10³ N·kg^-1)in China.According to the estimation of MPs abundance and sludge disposal data,the wastewater treatment plant will accumulate about 1.75×10¹² n MPs particles in sludge every year.Dewatered sludge has the highest MPs abundance(23.83±0.17×10³ N·kg^-1)and the highest proportion（12.59%）of small particle size MPs（<0.2 mm）of all organic solid wastes and recycling products in this study.The overall abundance of MPs in organic solid waste is:sludge>food waste>perishable domestic waste>pig manure>cow manure.The polymer risk index H of the perishable domestic waste with semi-dynamic composting,pig manure compost and sludge biogas residue are greater than 10.A small amount of polyvinyl chloride（PVC）copolymer and PMMA could contribute a lot to polymer risk.